[Objective] The air-gap magnetic field of permanent magnet generators is non-adjustable, resulting in a limited output voltage regulation range. To achieve an adjustable air-gap magnetic field, this paper studies a hybrid excitation DC generator with a tangential flux-concentrating parallel structure. [Methods] A Maxwell 2D and Simplorer field-circuit coupling model was built. The no-load characteristics and no-load air-gap flux density were simulated by finite element method. The parameter selection of related components in the filter circuit was discussed. The external characteristics were calculated, and the air-gap flux density and adjustment characteristics under armature reaction were analyzed. [Results] The simulation results showed that, under a constant external characteristic load, when the excitation current increased from -12 A to 0 A, the terminal voltage rose by 3.02 V. Further increasing the excitation current from 0 A to 12 A resulted in a terminal voltage increase of 24.66 V. Under armature reaction conditions, the amplitudes of the fundamental wave, 5P and 7P harmonics components of the air-gap flux density decreased by 0.05 T, 0.02 T, and 0.01 T, respectively, while the amplitudes of the 3P and 9P harmonics increased by 0.09 T and 0.03 T, respectively. The air-gap flux density curve exhibited a trend where one half was strengthened while the other half was weakened. [Conclusion] The no-load characteristics indicate that applying positive excitation to the generator is more effective than negative excitation, making it suitable for low-speed and high-torque applications. The output voltage increases with higher rotational speeds. The external characteristic curve shows that, under negative excitation current, the variation in the generator's phase voltage root mean square value is relatively small, whereas under positive excitation current, the variation is significantly larger. In the adjustment characteristics, to maintain a constant generator terminal voltage, the excitation current must be increased as the load current increases.